1. Field of the Invention
The present invention relates to an electrode for plasma etching, for example, used for a dry etching apparatus used in a process of fabricating a semiconductor device.
2. Description of the Related Art
As one of processes of fabricating a semiconductor device, there is a dry etching process that is one of processes of forming a circuit pattern on a silicon wafer. For example, when a parallel plate plasma etching apparatus is used, a silicon wafer to be treated is placed on a lower electrode, and RF Plasma is generated between the lower electrode and an upper electrode that is parallel thereto, having a lot of fine pores for passing of a reaction gas, so that the silicon wafer can be etched. In that case, the upper electrode itself is also etched by plasma while the silicon wafer is etched.
Conventionally, as shown in Japanese Patent. Application Laid-open (kokai) No.57-185982, the upper electrode has been made of graphite material highly purified, which is conductive and chemically stable, and scarcely contaminates silicon wafers with metal impurities. However, since the graphite material is complex of particles consisting of aggregate and matrix, there may be caused problems, such as a problem that constitutive particles drop therefrom due to plasma etching, resulting in large consumption of the material, a problem that the particles may drop on the silicon wafer to be treated, which may lead to trouble in formation of circuit pattern, or the like. To solve those problems, glassy carbon has been recently used as an upper electrode as shown in Japanese patent application laid open (Kokai) No.62-109317.
However, even if the glassy carbon is used as the upper electrode, the silicon wafer cannot be prevented from being contaminated. In fabrication of a semiconductor device, contamination of a silicon wafer may lead to degradation of performance, resulting in lowering of yield of the device. Accordingly, it has been proposed that a silicon single crystal that is the same as those used for a silicon wafer as a material of the upper electrode is used in place of a glassy carbon that may cause contamination, in Japanese patent application laid open (Kokai) No.62-85430, Japanese patent application laid open (Kokai) No.2-20018, Japanese patent application laid open (Kokai) No.6-177076, Japanese patent application laid open (Kokai) No.7-335635, Japanese patent application laid open (Kokai) No.9-129605, Japanese patent application laid open (Kokai) No.10-17393 or the like.
However, even if the upper electrode is made of single crystal silicon to keep high purity as a whole, adhesion of impurities into fine pores or the like is inevitable, since a silicon single crystal plate produced by slicing a single crystal silicon ingot is subjected to electrical discharge machining, ultrasonic machining, diamond drill machining or the like in order to make fine pores therein. Furthermore, adhesion of impurities from a polishing apparatus on the surface of the upper electrode during polishing is also inevitable. These are mainly heavy metal impurities, which are also generated from materials or tools used in each of processing apparatuses. Such heavy metal impurities may cause serious problems in a semiconductor device process, even if they exist in only slight amount. Namely, if the upper electrode to which impurities adhere is used without any treatment, the impurities drop on the silicon wafer. As a result, there may be caused a problem of lowering of yield of the semiconductor device fabricated on the silicon wafer.
Furthermore, such heavy metal impurities adhered to the upper electrode are not only adhered on the surface of the silicon electrode plate, but also diffused to an inner part thereof, resulting in generation of defects.
Such heavy metal impurities can be confirmed as small pits by breaking a silicon electrode plate to which impurities are adhered, subjecting the broken section to Secco etching or the like and observing it with a microscope.
Such defects due to impurities in a surface part of the silicon electrode plate may also cause problems while the silicon electrode plate is used. The problems will be hereunder explained more in detail. The silicon electrode plate itself is consumed on their surface as it is used, since it is also etched by plasma. In that case, defects due to impurities in a surface part of the silicon electrode plate are etched at high etching rate to form pits. Furthermore, temperature gets higher due to plasma in the part of the silicon electrode plate, the part of which is in contact with plasma. The small pits in the above-mentioned defect parts due to impurities will aggregate due to such high temperature, and become large pits that can be confirmed with the naked eye. If the silicon electrode plate is consumed, the large pits will be exposed. As a result, the surface of the plate will be in the state as if it is roughed. If the surface of the silicon electrode plate is roughed up, the figure of the roughness will be transcribed on the silicon wafer to be subjected to plasma etching. The transcribed figure of roughness may lead to difference in an etching rate, which may cause the lowering of yield of a device to be fabricated on the silicon wafer.
The present invention has been accomplished to solve the above-mentioned problems, and an object of the present invention is to provide a silicon electrode plate made of silicon single crystal that is used as an upper electrode in a plasma etching apparatus wherein problems due to adhesion of impurities such as heavy metal or the like can be prevented.
To achieve the above object, the present invention provides a silicon electrode plate comprising silicon single crystal used as an upper electrode in a plasma etching apparatus wherein concentration of interstitial oxygen contained in the silicon electrode plate is not less than 5xc3x971017 atoms/cm3 and not more than 1.5xc3x971018 atoms/cm3.
As described above, when a silicon electrode plate wherein concentration of interstitial oxygen contained in the silicon electrode plate is not less than 5xc3x971017 atoms/cm3 and not more than 1.5xc3x971018 atoms/cm3 is used, for example, as an upper electrode in a plasma etching apparatus, oxygen is precipitated in a sufficient amount in a bulk part of the electrode plate at a high temperature. As a result, so-called intrinsic gettering effect (IG effect) wherein impurities such as harmful heavy metal adhered to the silicon electrode plate are captured can be sufficiently achieved. Accordingly, the electrode plate has a gettering effect, and there can be prevented problems such as a drop of the adhered impurities on a silicon wafer to be treated. Furthermore, defects in a surface part of the silicon electrode plate can be controlled by the gettering effect, so that roughness of the surface can be reduced. Moreover, if oxygen concentration is in the above-mentioned range, defects due to oxygen precipitation such as OSF (oxidation induced stacking fault) which excessive oxygen precipitation causes during use of the plate are not caused. As a result, roughness of the surface of the electrode plate caused thereby can also be prevented. Accordingly, the surface roughness is never transcribed on the silicon wafer to be treated.
In that case, nitrogen concentration in the silicon electrode plate is preferably not less than 5xc3x971013 atoms/cm3 and not more than 5xc3x971015 atoms/cm3.
As described above, if nitrogen concentration in the silicon electrode plate is not less than 5xc3x971013 atoms/cm3 and not more than 5xc3x97105 atoms/cm3, oxygen precipitation in a bulk part of the silicon electrode plate is suitably promoted, and thus the gettering effect can be more effective.
In that case, the surface of the silicon electrode plate is preferably subjected to etching treatment.
As described above, if the surface of the silicon electrode plate is subjected to etching treatment, a work damage layer generated during manufacturing and processing of the silicon electrode plate can be removed, and surface roughness of the silicon electrode plate can be removed. As a result, a problem that figure of the roughness is transcribed on the silicon wafer to be treated can be prevented.
As described above, according to the present invention, since concentration of interstitial oxygen contained in a silicon electrode plate made of silicon single crystal used as an upper electrode in a plasma etching apparatus is not less than 5xc3x971017 atoms/cm3 and not more than 1.5xc3x971018 atoms/cm3, a sufficient gettering effect is afforded to the silicon electrode plate. Accordingly, disadvantages due to impurities such as heavy metal or the like can be prevented, and yield in production of a semiconductor device can be improved.